It is known that in densitometry, the size of the measured spot, i.e. the area of the measured object which is illuminated by the light source, determines the size of the smallest detectable detail and consequently the resolution. To obtain high resolution, attempts are made to keep the measured spot very small. However in known densitometers, which illuminate the measured spot with constant light intensity over the entire area, there are limits to the extent to which the spot can be reduced because the inaccuracy of measurement increases with resolution which arises because the result will be increasingly dependent on the position of the spot relative to the measured object. This applies particularly to the measurement of raster or pattern-like objects, e.g offset printing or the like, where even small changes in the position of the spot can result in considerable differences in the result.
The object of the invention, therefore, is to provide a measuring head of a densitometer so as to enable a smaller spot to be used and consequently higher resolution to be obtained with the same or even better accuracy or reliability of results.
As will be shown hereinafter this improvement is obtained by employing in accord with the invention a particular light intensity distribution over the spot area. There is disclosed a filter for providing the desired distribution which filter comprises a combination of lenses of transparent and neutral grey glass. British Patent Specification No. 982,520 discloses a non-refractive combination of a convex and concave lens, one of the two lenses being made of neutral-grey glass. The purpose of this combination of lenses is to compensate the known Cos.sup.4 light loss in an objective, more particularly when used in TV systems. This publication does not disclose anything relating to the problem on which the present invention is based.